Dark-field mid-infrared photothermal microscopy

1. A wide-field mid-infrared photothermal microscopy (MIP) system for analyzing a sample on a substrate using dark-field illumination, comprising:

a mid-infrared (IR) optical source configured to generate a mid-infrared beam, the mid-infrared beam being directed at the sample to heat the sample;

a visible light source for generating a light, the light illuminating the sample on the substrate and creating a scattered field and a reflected field along a collection path of the system,

wherein the MIP system is configured to simultaneously heat the sample with the mid-infrared optical source while imaging the sample with the visible light source;

a pupil mask positioned along the collection path to block the reflected field while allowing the scattered field to pass therethrough, wherein the pupil mask centrally houses a dot blocker to block the reflected field; and

a camera positioned at an end of the collection path to collect the scattered field and generate a dark-field image of the sample.

2. The system of claim 1 , wherein the collection path includes two identical achromatic doublet lenses through which the light illuminating the sample on the substrate passes.

3. The system of claim 2 , wherein the collection path includes a high numerical aperture objective lens.

4. The system of claim 3 , wherein the collection path includes a CMOS imaging sensor for detecting light along the collection path.

5. The system of claim 1 , wherein the pupil mask is configured to adjust specularly reflected light for optimization of interferometric signal.

6. The system of claim 1 , wherein the pupil mask provides quasi-dark illumination of wavelength size particles while maintaining detector at shot-noise-limit operation so that no reflected light from the substrate reaches the visible light source.

7. The system of claim 1 , wherein the pupil mask provides a photothermal effect broadening the angular distribution of radiation.

8. The system of claim 7 , wherein the broadening of the angular distribution of the radiation yields a lower directivity compared to dc signal.

9. The system of claim 1 , wherein the pupil mask blocks 1:1000 of the reflected light by being placed into a Fourier plane.

10. The system of claim 9 , wherein an absorptive material is deposited at the center of an optic quality quartz disposed on the pupil mask.

11. The system of claim 1 , wherein the dot blocker filters the reflected illuminating light from the substrate.

12. The system of claim 11 , wherein the dot blocker has a diameter of 1.6 mm.

13. The system of claim 12 , wherein the dot blocker blocks 6% of a pupil while passing a large fraction of collected scattered light.

14. The system of claim 13 , wherein a photothermal collected power drop is 11% for the 1.6 mm blocker.

15. The system of claim 14 , wherein the system for analyzing a sample on a substrate using dark-field illumination can be implemented on most of the standard bright-field objectives.

16. The system of claim 1 , wherein the illuminating light reflected from the substrate and refocused at the objective back pupil allows for access to the reflected light at a conjugate plane.

17. The system of claim 1 , wherein the visible light source includes at least one of a narrow-band light source, an LED light source, a monochromatic light source, a laser light source and a visible light source.

18. A wide-field mid-infrared photothermal microscopy (MIP) system for analyzing a sample on a substrate using dark-field illumination, comprising:

a mid-infrared (IR) optical source configured to generate a mid-infrared beam, the mid-infrared beam being directed at the sample to heat the sample;

a visible light source for generating a light, the light illuminating the sample on the substrate and creating a scattered field and a reflected field along a collection path of the system, wherein the scattered field is refocused at an objective back pupil conjugate with a focal plane,

wherein the MIP system is configured to simultaneously heat the sample with the mid-infrared optical source while imaging the sample with the visible light source;

a pupil mask positioned along the collection path to block the reflected field while allowing the scattered field to pass therethrough, wherein the pupil mask centrally houses a dot blocker to block the reflected field; and

a camera positioned at an end of the collection path to collect the scattered field and generate a dark-field image of the sample.

19. A wide-field mid-infrared photothermal microscopy (MIP) method for analyzing a sample on a substrate using dark-field illumination, comprising:

generating a mid-infrared beam from a mid-infrared (IR) optical source, the mid-infrared beam being directed at the sample to heat the sample;

generating a light from a visible light source for, the light illuminating the sample on the substrate and creating a scattered field and a reflected field along a collection path of the system,

wherein the MIP system is configured to simultaneously heat the sample with the mid-infrared optical source while imaging the sample with the visible light source;

blocking the reflected field with a pupil mask positioned along the collection path while allowing the scattered field to pass therethrough, wherein the pupil mask centrally houses a dot blocker to block the reflected field; and

collecting the scattered field with a camera positioned at an end of the collection path and generating a dark-field image of the sample.